Pyrochlore phases in the system ZnOBi2O3Sb2O5: II. Crystal structures of Zn2Bi3.08Sb2.92O14+δ and Zn2+xBi2.96−(x−y)Sb3.04−yO14.04+δ

Rietveld refinement of combined X-ray and neutron diffraction data has, within errors, confirmed the stoichiometries of two, cubic pyrochlore phases in the ZnOBi₂O₃Sb₂O₅ system. Neither phase has the ‘ideal’ stoichiometry, Zn₂Bi₃Sb₃O₁₄. One phase, P₁, is a Zn-rich, Bi-deficient solid solution Zn_2+xBi_{2.96−(x−y)}Sb_3.04−yO_14.04+δ. The other, P₂, is a Bi-rich line phase, stoichiometry Zn₂Bi_3.08Sb_2.92O_14+δ. Both structures have a mixture of Bi, Zn on the A-sites and Zn, Sb on the B-sites. However, Zn is displaced off-centre in the A-sites to achieve lower co-ordination number with realistic ZnO bond lengths. Additional structural complexities arise from: displacement of O(2) atoms; partial occupancies of O(1) and O(2) sites; partial occupancy of a third, interstitial oxygen site, O(3). Since the multiplicities of the off-centre sites are much higher than those of the ideal positions, there is considerable possibility for correlated short range order throughout the structures.